FIG. 7A is a schematic diagram of a projection display device of the prior art. FIG. 7B is an enlarged cross-sectional view of a light source section of the projection display device shown in FIG. 7A.
As illustrated, a light source section 1 is comprised of a light source 2 which may be a metal halide lamp emitting white light, and a parabolic reflector or mirror 3, which is concave and which reflects light radiated from the light source 2 and directs that light as a luminous flux of substantially parallel rays and of a circular cross section.
The light source 2 comprises a pair of discharge electrodes 4, between which a luminescent section 7 of the light source is formed. The electrodes 4 are supported by a transparent outer wall 6 made for example of quartz glass. Light source 2 is placed in such a manner that luminescent section 7 constituting a point light source is near the focus of the parabolic mirror 3. A power source 5 is for driving the light source 2.
The light source section 1 emits a luminous flux 8 which travels substantially in parallel, and has a circular cross-section. A dichroic mirror 9 has such a wavelength selectivity that red light flux component 13R of luminous flux 8 is reflected through a substantially right angle (with the angle between the incident and reflected lights being about 90.degree.) while a blue and a green flux components pass through unhindered as luminous flux 10. A reflective mirror 11 reflects red light flux component 13R through a substantially right angle. The luminous flux after the reflection at the reflective mirror 11 is denoted by 14R.
Another dichroic mirror 12 has such a wavelength selectivity that blue light flux component 14B of luminous flux 10 is reflected through a substantially right angle while a green flux component passes through unhindered as luminous flux 14G.
The dichroic mirrors 9 and 12 serve to separate light flux from the illumination system into multiple fluxes of differing wavelength ranges.
Transmission-type light valves 15R, 15B, and 15G perform two-dimensional intensity modulation of luminous fluxes 14R, 14B, and 14G, respectively, at surfaces parallel to those fluxes. The light valves 15R, 15B and 15G modulate the respective fluxes of differing wavelength ranges, and have cross sections identical with each other. They could be, for example, image display panels which utilize the optoelectronic properties of liquid crystals and whose frames are normally rectangular in shape. The ratio of height to width would be 3:4 for the NTSC format; and, 9:16 for the high-definition television (HDTV) format. These light valves are driven by TV signals or the like. In other words, the light intensity is modulated along the surface of the panel, i.e., subjected to space or two-dimensional modulation. The diagonal screen size of a light valve is usually somewhere between one to five inches.
Luminescent fluxes that have been two-dimensional intensity modulated by the transmission-type light valves 15R, 15B, and 15G are denoted by 16R, 16B, and 16G, respectively.
A mirror 17 reflects flux component 16G through a substantially right angle. The luminous flux after the reflection at the mirror 17 is denoted by 18G. A dichroic mirror 19 passes the flux component 16R unhindered, and reflects the flux component 16B through a substantially right angle, and which thereby merges flux components 16R and 16B to form luminous flux 20. Likewise, another dichroic mirror 21 passes luminous flux 20 unhindered, and reflects flux component 18G through a substantially right angle, and which thereby merges luminous flux 20 and flux component 18G to form luminous flux 22. In summary, luminous flux 22 is therefore a flux created by merging red, blue, and green flux components after they have been intensity-modulated at their respective light valves.
A projection lens 23 projects luminous flux 22 onto a screen 24: that is, enlarged real images of the image on the light valves 15R, 15B, and 15G are formed on the screen 24. This diagonal screen size is generally somewhere in the range of 20 to 200 inches.
The light valves 15R, 15B, and 15G are all located at an equal distance (distance along the optical path) from the projection lens. In addition, the cross-sectional area of the luminous flux shone into light valves 15R, 15B, and 15G is made slightly larger than that of the light valves themselves.
FIG. 8A and FIG. 8B are diagrams for explaining the action of a prior-art light source device. Let us consider light beams 35a, 35b, and 35c, which are, before being reflected off parabolic mirror 36, all on the same cone described (with the luminescent section 7 as the vertex) by the angle .theta. (relative to the optical axis), and have an angle of rotation (around the optical axis and relative to the x-axis) of .phi..sub.a, .phi..sub.b, and .phi..sub.c, respectively.
Upon being reflected off parabolic mirror 36, light beams 35a, 35b, and 35c all become parallel to and equidistant from optical axis 37. It will thus be appreciated that the assembly of all the beams emanating from the light source and reflected at the parabolic mirror 36 which is disk-shaped as viewed in the direction of the optical axis 37 form a luminous flux 38 having a circular cross section.
Returning to FIG. 7A, white light emitted from the light source 2 is reflected off parabolic mirror 3, thereby creating a flux which has a circular cross section and which travels substantially parallel. This flux is broken down into red, blue, and green flux components 14R, 14B, and 14G by dichroic mirrors 9 and 12. The flux components, in which the light is substantially parallel, are two-dimensional intensity modulated by their respective light valves 15R, 15B, and 15G. Images which appear on the light valves are projected, being enlarged, onto the screen 24 by the projection lens 23, thereby allowing a viewer to watch an enlarged TV video image or the like.
Since a prior-art projection display device is configured in a manner described above, of the circular cross section of luminous fluxes 14R, 14G, and 14B, part only luminous flux 25 falling on rectangular light valves 15R, 15G, and 15B (white section of FIG. 9) is utilized; the remaining luminous flux portion 26 (hatched section of FIG. 9) is discarded. For the NTSC format, that is, for a height to width ratio of 3:4, a fairly small proportion (61%) of the luminous flux is utilized even when the size of the circular flux is such that it just covers the corners of the light valve. The remainder (39%) of the luminous flux is discarded (not utilized). There are several problems with this approach. One is that, even if a metal halide lamp (which has a light emitting efficiency that is one of the highest among the light sources currently available) is used, the brightness of such a projection display is not as good as that of a CRT; therefore, it is necessary that the efficiency of light source utilization be increased as much as possible.